Matrix Chains Research Articles

Nanoparticle Dispersion and Glass Transition Behavior of Polyimide-grafted Silica Nanocomposites

How to control the spatial distribution of nanoparticles to meet different performance requirements is a constant challenge in the field of polymer nanocomposites. Current studies have been focused on the flexible polymer chain systems. In this study, the rigid polyimide (PI) chain grafted silica particles with different grafting chain lengths and grafting densities were prepared by “grafting to” method, and the influence of polymerization degree of grafted chains (N), matrix chains (P), and grafting density (σ) on the spatial distribution of nanoparticles in the PI matrix was explored. The glass transition temperature (Tg) of PI composites was systematically investigated as well. The results show that silica particles are well dispersed in polyamic acid composite systems, while aggregation and small clusters appear in PI nanocomposites after thermal imidization. Besides, the particle size has no impact on the spatial distribution of nanoparticles. When σ·N0.5 ≪ (N/P)2, the grafted and matrix chains interpenetrate, and the frictional resistance of the segment increases, resulting in restricted relaxation kinetics and Tg increase of the PI composite system. In addition, smaller particle size and longer grafted chains are beneficial to improving Tg of composites. These results are all propitious to complete the microstructure control theory of nanocomposites and make a theoretical foundation for the high performance and multi-function of PI nanocomposites.

Construction of particle network for ultrahigh permittivity of dielectric polymer composite toward energy devices: A molecular dynamics study

Energy storage and energy harvesting are the effective strategies to solve the energy and environmental crises. Dielectric polymer composites with high permittivity and energy density are extremely desired for the energy devices. However, there is a great deviation of effective permittivity between the real polymer composite and the ideal model, limiting their applications. In the present work, we adopt the coarse-grained molecular dynamics simulations to study the construction of particle network and the structure–property relationship, aiming to design polymer composite with high effective permittivity at low particle loading. Based on the analysis of topological structure of particle network, we propose three strategies, including grafting polymer chains to particles, selective locating into diblock copolymer and tuning moderate particle–matrix interaction. In particular, if the matrix chains are directly grafted to the particles, the network forms at very low particle loading, i.e., very low percolation threshold is achieved. Constructing the particle network opens a new way to design high permittivity of dielectric polymer composite, which is essential for their practical applications such as dielectric capacitors and triboelectric nanogenerators.

Tough and self-healable nanocomposite hydrogels from poly(acrylic acid) and polyacrylamide grafted cellulose nanocrystal crosslinked by coordination bonds and hydrogen bonds

Developing self-healable hydrogels with excellent mechanical performance is important for their potential applications. In this paper, tough and self-healable dual physically crosslinked nanocomposite hydrogels reinforced with polyacrylamide grafted cellulose nanocrystal (CNC-g-PAM) were fabricated by in situ free radical polymerization of acrylic acid (AA) monomers in the presence of FeCl3 (0.75% of AA mole) and CNC-g-PAM (1–3% of AA mass). The primary network of PAA/CNC-g-PAM nanocomposite hydrogels was constructed via coordination bonds between carboxylic groups from PAA and Fe3+ ions, while CNC-g-PAM served as both reinforcing nanofillers and physical crosslinkers via hydrogen bonds between PAA matrix and PAM chains on the surface of CNC. The nanocomposite hydrogels exhibited simultaneously improved elastic modulus, tensile strength and elongation at break with the increase of CNC-g-PAM loading. The dynamic nature of both coordination bonds and hydrogen bonds endowed the nanocomposite hydrogels with excellent energy dissipation, self-recovery ability and autonomous self-healing capacity.

Interacting chains of orbital polarons in the colossal magnetoresistance material La1−xSrxMnO3 revealed by spin and lattice dynamics

The origin of the effect of colossal magneto-resistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudo-cubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5) and various temperatures and report a new lattice dynamics study at x0=0.2, representative of the optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum Enmag (q) characteristic of the internal excitations of "orbital polarons" defined by Mn3+ neighbors surrounding a Mn4+ center with a hole. Because of its hopping, the hole mixes up dynamically all the possible orbital configurations of its surrounding Mn3+ sites with degenerate energies. The Enmag values indicate a lift of orbital degeneracy by phonon excitations. The number n varies with the spatial dimension D of the polaron and the q-range determines its size. At x=0.125 and x=0.3 the spectrum reveals 2D polarons coupled by exchange and 3D "free" polarons respectively, with sizes l=1.67a < 2a in all bond directions. At x0=0.2, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size l=2a with a periodic distribution over ~ 3a and an interaction energy ~ 3 meV. At T < Tc the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a non-metallic matrix may be crucial for CMR.

The Effect of Uniaxial Extrusion on the Degree of Reinforcement of Nanocomposites Polyvinyl Chloride/Boron Nitride

The influence of uniaxial extrusion on the structure, molecular characteristics, and properties of polymer/2D nanofiller nanocomposites is elucidated by example of polyvinyl chloride/boron nitride nanocomposites. The extrusion is found to cause a drastic (by 2.5 times) increase in statistical rigidity of polymer chains in the nanocomposite matrix determined by their characteristic ratio. The characteristic ratio for unoriented samples is less than that for a polymer precursor, but exceeds the values for oriented specimens. Nanofiller particles (or their aggregates) form a fractal network (“skeleton”) in the polymer matrix, whose dimension increases upon uniaxial extrusion. This effect leads to a linear reduction of the percolation index in the percolation model of reinforcement of nanocomposites, which significantly affects the degree of their reinforcement. When the dimension reaches its threshold of 2.8, the percolation index changes values from positive to negative values, meaning the transition from the “ant” to the “termite” limit. This yields an abrupt (to ~150) increase in the degree of reinforcement of polymer nanocomposites at low nanofiller contents. The characteristic ratio of polymer chains of the nanocomposite matrix is closely related to the aggregation level of the 2D nanofiller, which depends on the number of boron nitride slabs in a “package” (tactoid). The relative fraction of interfaces serving as reinforcing elements in a nanocomposite structure by analogy with nanofillers is also a function of statistical rigidity of polymer chains in the matrix and increases with the characteristic ratio. The theoretical estimates in the context of the percolation model of reinforcement reveal the prospects in the improvement of the properties of polymer/2D nanofiller nanocomposites.

Understanding the Static Interfacial Polymer Layer by Exploring the Dispersion States of Nanocomposites.

The dynamic and static properties of the interfacial region between polymer and nanoparticles have wide-ranging consequences on performances of nanomaterials. The thickness and density of the static layer are particularly difficult to assess experimentally due to superimposing nanoparticle interactions. Here, we tune the dispersion of silica nanoparticles in nanocomposites by preadsorption of polymer layers in the precursor solutions, and by varying the molecular weight of the matrix chains. Nanocomposite structures ranging from ideal dispersion to repulsive order or various degrees of aggregation are generated and observed by small-angle scattering. Preadsorbed chains are found to promote ideal dispersion, before desorption in the late stages of nanocomposite formation. The microstructure of the interfacial polymer layer is characterized by detailed modeling of X-ray and neutron scattering. Only in ideally well-dispersed systems a static interfacial layer of reduced polymer density over a thickness of ca. 2 nm is evidenced based on the analysis with a form-free density profile optimized using numerical simulations. This interfacial gradient layer is found to be independent of the thickness of the initially adsorbed polymer, but appears to be generated by out-of-equilibrium packing and folding of the preadsorbed layer. The impact of annealing is investigated to study the approach of equilibrium, showing that initially ideally well-dispersed systems adopt a repulsive hard-sphere structure, while the static interfacial layer disappears. This study thus promotes the fundamental understanding of the interplay between effects which are decisive for macroscopic material properties: polymer-mediated interparticle interactions, and particle interfacial effects on the surrounding polymer.

Constraint Release Mechanisms for H-Polymers Moving in Linear Matrices of Varying Molar Masses

We investigate the influence of the environment on the relaxation dynamics of well-defined H-polymers diluted in a matrix of linear chains. The molar mass of the linear chain matrix is systematically varied and the relaxation dynamics of the H-polymer is probed by means of linear viscoelastic measurements, with the aim to understand its altered motion in different blends, compared to its pure melt state. Our results indicate that short unentangled linear chains accelerate the relaxation of both the branches and the backbone of the H-polymers by acting as an effective solvent. On the other hand, the relaxation of the H-polymer in an entangled matrix is slowed-down, with the degree of retardation depending on the entanglement number of the linear chains. We show that this retardation can be quantified by considering that the H-polymers are moving in a dilated tube at the rhythm of the motion of the linear matrix.

Impact of Hydrogen Bonding Interactions on Graft–Matrix Wetting and Structure in Polymer Nanocomposites

We present a new coarse-grained (CG) model that captures directional interactions between graft and matrix polymer chains in polymer nanocomposites (PNCs) comprising polymer grafted spherical nanoparticles in a matrix polymer. In this CG model we incorporate acceptor and donor CG beads along with graft and matrix monomer CG beads and optimize the bonded and nonbonded interactions to mimic directional and specific H-bonding between the acceptor and donor sites on graft and matrix chains, respectively. Using this CG model and molecular dynamics simulations we show that H-bonding interactions between graft and matrix polymer chains increase the grafted layer wetting by matrix chains compared to that at the purely entropic limit. One can achieve equivalent grafted layer wetting in PNCs with directional acceptor–donor interactions and PNCs with isotropic graft–matrix interactions, but the directional acceptor–donor interaction strength needs to be much stronger than the isotropic graft–matrix monomer attractio...

Revisiting KAI theory and its application to mixed composite system “–PVA” fabricated at moderate elevated temperature

Ishibashi and Takagi considered ‘ferroelectric domain switching’ just equivalent to ‘phase evolution during crystal growth’. They slightly modified Kolmogorov and Avrami’s classical theory of ‘phase transformations in crystal growth’ and applied it to the switching data in ferroelectrics. Here an effort is made to elucidate mathematical steps in evolution of this new KAI model used to fit experimental switching data to estimate microscopic switching parameters. In mixed composite system (x = 0.09), the domain growth dimension () is found to expand from to when signal amplitude changed from to The cesium substitution is supposed to have enabled lone pairs on central nitrogen of nitrite group to orient constituting branches into more ordered state in the matrix of molecular chains of the PVA facilitating control of crystallization process which led to lowering of strain giving rise to the increased change in with change in pulse amplitude.

Diffusion of polymer-grafted nanoparticles in a homopolymer matrix.

Molecular dynamics simulations are used to study the diffusion of polymer-grafted nanoparticles (PGNPs) in polymer. The diffusivity of PGNPs in the homopolymer matrix is investigated as a function of graft length and grafting density, and it is compared to that of bare nanoparticles with comparable effective size. Our results indicate that, in addition to the increase in the effective size of PGNPs due to grafting, the interpenetration of matrix polymers into the grafted layer also plays an important role in the mobility of PGNPs. In systems consisting of both PGNPs and bare particles, the spatial arrangement of the bare particles was found to be having a significant influence on the mobility of PGNPs. At low graft length and high grafting density, the matrix chains dewets the grafted layer, due to autophobic dewetting, creating a sharper interface between the matrix and the grafted layer. The bare particles then migrate to the interface creating a barrier around the PGNPs that hinders the matrix-graft interpenetration and results in the higher mobility of PGNPs. Our results emphasize the importance of polymer-particle interface on the dynamic properties of polymer nanocomposites.

Simulation and verification of pulsed laser beam propagation underwater using Markov chains [Invited

One fast simulation method using Markov chains was introduced to simulate angular, energy, and temporal characteristics of pulsed laser beam propagation underwater. Angular dispersion of photons with a different number of collisions was calculated based on scattering function and the state transition matrix of Markov chains. Temporal distribution and energy on the receiving plane were obtained, respectively, by use of a novel successive layering model and receiving ratio. The validity of this method was verified by comparing it with the Monte Carlo ray tracing (MCRT) method. The simulation results were close to those obtained by MCRT but were less time consuming and had smoother curves.

Molecular dynamics simulation study of the fracture properties of polymer nanocomposites filled with grafted nanoparticles.

By employing coarse-grained molecular dynamics simulations, we investigated the fracture behavior of polymer nanocomposites (PNCs) filled with polymer-grafted nanoparticles (NPs) in detail by particularly regulating the grafting density and the length of the grafted chain. By calculating their fracture energy, we observed that their rupture properties first increase and then decrease with the increase of the grafting density or the length of the grafted chains. Their bond orientation degree and their van der Waals energy change are characterized to understand their fracture behavior. To further explain it, we analyzed the contributions of the matrix chains, grafted chains, and NPs to the total stress. It is interesting to find that the stress borne by one bead of matrix chains or NPs gradually increases with the grafting density, while the stress borne by the grafted chains first increases and then decreases. In addition, the stress borne by one bead of matrix chains or grafted chains gradually increases with the length of the grafted chains, while the stress borne by NPs remains nearly unchanged. As a result of these contributions, the optimal fracture properties appear at the moderate grafting density or length of the grafted chain. Then, the number of voids is quantified, which first increases and then decreases with strain because of the coalescence of small voids into large ones. Accompanying this, the maximum void size increases significantly. Furthermore, the maximum number of voids increases with the grafting density, while it is nearly independent of the length of the grafted chain. In particular, the voids are preferably generated at the end beads of the chains or at the surfaces of the NPs. In summary, this work could provide some further understanding of how the grafted chains affect the fracture properties of the PNCs.

Noncontact Imaging of Ion Dynamics in Polymer Electrolytes with Time-Resolved Electrostatic Force Microscopy.

Ionic-transport processes govern performance in many classic and emerging devices, ranging from battery storage to modern mixed-conduction organic electrochemical transistors (OECT). Here, we study local ion-transport dynamics in polymer films using time-resolved electrostatic force microscopy (trEFM). We establish a correspondence between local and macroscopic measurements using local trEFM and macroscopic electrical impedance spectroscopy (EIS). We use polymer films doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI) as a model system where the polymer backbone has oxanorbornenedicarboximide repeat units with an oligomeric ethylene oxide side chain of length n. Our results show that the local polymer response measured in the time domain with trEFM follows stretched-exponential relaxation kinetics, consistent with the Havriliak-Negami relaxation we measure in the frequency-domain EIS data for macroscopic samples of the same polymers. Furthermore, we show that the trEFM results capture the same trends as the EIS results-changes in ion dynamics with increasing temperature, increasing salt concentration, and increasing volume fraction of ethylene oxide side chains in the polymer matrix evolve with the same trends in both measurement techniques. We conclude from this correlation that trEFM data reflect, at the nanoscale, the same ionic processes probed in conventional EIS at the device level. Finally, as an example application for emerging materials syntheses, we use trEFM and infrared photoinduced force microscopy (PiFM) to image a diblock copolymer electrolyte for next-generation solid-state energy storage applications.

Selective location and migration of poly(methyl methacrylate)- grafted clay nanosheets with low grafting density in poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends

The location and migration of poly (methyl methacrylate)-grafted clay (clay-g-PMMA) nanosheets with low grafting density in a phase-separated polymer blend of poly (methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) was investigated. The grafting density and molecular weight for grafted PMMA chains on the surface of clay nanosheets was about 0.04 chains/nm2 and 7.8 × 104, respectively, indicating that the grafted chains might exhibit the mushroom-like distribution. With phase separation of PMMA/SAN blend matrix, the well-dispersed clay-g-PMMA nanosheets in the blend matrix were first located in the SAN-rich phase and then gradually migrated to the interface between PMMA and SAN. The first abnormal location of modified clay in the blend matrix might be attributed to the favorable interaction between clay-g-PMMA and SAN induced by the miscibility of grafted PMMA/surrounding SAN chains and the unfavorable interaction between clay-g-PMMA and PMMA without any interpenetration of grafted and matrix chains. The enhanced miscibility between grafted PMMA and surrounding SAN just began to be destroyed after being annealed for a long time, resulting in the final migration of clay-g-PMMA to the interface of PMMA/SAN (thermodynamic equilibrium state).

Towards polylactide/core-shell rubber blends with balanced stiffness and toughness via the formation of rubber particle network with the aid of stereocomplex crystallites

As the most promising bio-derived and biodegradable polymers with tremendous application potential, the practical application of poly(l-lactide) (PLLA) has been severely restricted by its inherent brittleness. Unfortunately, blending with various rubbers can substantially enhance the toughness of PLLA, but usually causes a significant deterioration in mechanical strength and modulus. In this work, taking novel PLLA/core-shell rubber (CSR) blends as an example, we describe a facile and robust strategy to obtain good stiffness-toughness balance by promoting the networking of CSR particles within PLLA matrix with the aid of stereocomplex (SC) crystallites. To do this, small amounts (5–20 wt%) of poly(d-lactide) (PDLA) were incorporated into the blends through simple melt-blending. The results demonstrate that the CSR particles can be uniformly dispersed in PLLA matrix due to the good interfacial compatibility, but the stereocomplex (SC) crystallites formed between the incorporated PDLA and PLLA matrix chains induces the organization of the CSR particles into a network-like structure in the matrix. Compared with the well-dispersed structure, the network-like structure can endow the PLLA/CSR blends with dramatically enhanced impact toughness, without compromising their mechanical strength and modulus, indicating a marvelous balance between the stiffness and toughness. Furthermore, the elongation at break of the blends is also enhanced greatly. These interesting findings suggest that the formation of SC crystallites could provide a promising avenue for fabricating CSR particles toughened PLLA blends with balanced stiffness-toughness.

Interfacial interactions in PMMA/silica nanocomposites enhance the performance of parts created by Fused Filament Fabrication

As an additive manufacturing method, Fused Filament Fabrication (FFF) is conceptually attractive due to its agility and adaptability. However, FFF-printed parts exhibit poor mechanical properties as compared to parts manufactured by traditional methods. Here, the addition of silica nanoparticles (Si NPs) into FFF-printed parts is demonstrated to markedly improve a variety of thermomechanical properties. Specifically, dynamic mechanical analysis (DMA) and tensile tests indicate that the glass transition temperature, Young's modulus, elongation at break, ultimate tensile strength, and storage and loss moduli all increase with the Si NP loading. Small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) demonstrate similar hierarchical structures in all FFF-printed nanocomposites, which suggests that improvements in material properties with Si NP loading are likely due to an increasing number of hydrogen bonding interactions between PMMA matrix chains and hydroxyl groups on the Si NP surface. The potential of nanocomposite filaments to increase the thermomechanical properties of FFF-printed specimens provides a facile route to overcome limitations in FFF-printed part strength by combining the properties of hard and soft matter.

Glass transition broadening via nanofiller‐contiguous polymer network in aromatic thermosetting copolyester nanocomposites

ABSTRACTThe glass transition is a genuine imprint of temperature‐dependent structural relaxation dynamics of backbone chains in amorphous polymers, which can also reflect features of chemical transformations induced in macromolecular architectures. Optimization of thermophysical properties of polymer nanocomposites beyond the state of the art is contingent on strong interfacial bonding between nanofiller particles and host polymer matrix chains that accordingly modifies glass transition characteristics. Contemporary polymer nanocomposite configurations have demonstrated only marginal glass transition temperature shifts utilizing conventional polymer matrix and functionalized nanofiller combinations. We present nanofiller‐contiguous polymer network with aromatic thermosetting copolyester nanocomposites in which carbon nanofillers covalently conjugate with cure advancing crosslinked backbone chains through functional end‐groups of constituent precursor oligomers upon an in situ polymerization reaction. Via thoroughly transformed backbone chain configuration, the polymer nanocomposites demonstrate unprecedented glass transition peak broadening by about 100 °C along with significant temperature upshift of around 80 °C. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1595–1603

Assessing risk through composite FMEA with pairwise matrix and Markov chains

PurposeFailure mode and effect analysis (FMEA) is an analysis technique for identifying and eliminating known and/or potential failures and problems from products, processes or systems. Notwithstanding its diffusion, traditional FMEA has several limitations. Lately, scientific research has been focused on improving said limitations, yet the process is still ongoing. The purpose of this paper is to support developments in this area.Design/methodology/approachThe paper improves the conventional FMEA by using the method of pairwise comparison to establish the relative importance of the input factors in risk priority number calculation, and Markov chains to calculate risk distributions in the long term.FindingsThe functioning and usefulness of the proposed methodology is demonstrated through an application to the construction industry, one of the world’s biggest industrial sectors, dogged by a high rate of work-related injuries and casualties.Originality/valueHaving demonstrated the applicability of the novel methodology to a real domain, the paper contributes to the process of overcoming traditional FMEA limitations.

Surface-modified microcrystalline cellulose for reinforcement of chitosan film

We here report an inexpensive active filler of surface-modified microcrystalline cellulose (MCC) particle for reinforcement of polymer material. A commercial MCC powder with particle size of about 50 μm was mixed with urea and was then irradiated under microwave for the modification. The obtained urea-modified MCC (u-MCC) was able to be dispersed in a chitosan solution without precipitation for over 48 h, which provided a time window for its processing. After the mixture slurry was cast onto glass substrate and then coagulated in a NaOH aqueous solution, a full natural polymer composite film has been prepared. The mechanical properties of the obtained composite films have been carefully measured and discussed. Our results indicate that the mechanical properties reach the highest when the urea-modified MCC is 7 wt% in the composite film: the tensile strength, the breaking elongation, the Young’s modulus and the fracture energy are respectively about 2.0, 2.1, 2.4 and 6.0 times those of the pure chitosan film. The reinforcement of the composite material with the u-MCC particles was studied through scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction analyses, and has been attributed to the strong interaction between the surface-modified MCC particles and the neighboring chitosan chains in the film matrix. Moreover, the water vapor permeability and the transparency of the composite films have been determined to evaluate the potential applications such as gas impervious material and packaging material.

In-Situ Preparation of Aramid-Multiwalled CNT Nano-Composites: Morphology, Thermal Mechanical and Electric Properties.

In this work in-situ polymerization technique has been used to chemically link the functionalized multiwalled carbon nanotubes (CNTs) with aramid matrix chains. Phenylene diamine monomers were reacted in the first stage with the carboxylic acid functionalized CNTs and then amidized in-situ using terephthaloyl chloride generating chemically bonded CNTs with the matrix. Various proportions of the CNTs were used to prepare the hybrid materials. The functionalization procedure was studied by Fourier transform infrared (FTIR) spectroscopy and composite morphology investigated by scanning electron microscopy (SEM). Thermal mechanical properties of these hybrids, together with those where pristine CNTs with similar loadings were used, are compared using tensile and dynamic mechanical analysis (DMA). The tensile strength and temperature involving α-relaxations on CNT loading increased with CNT loading in both systems, but much higher values, i.e., 267 MPa and 353 °C, respectively, were obtained in the chemically bonded system, which are related to the nature of the interface developed as observed in SE micrographs. The water absorption capacity of the films was significantly reduced from 6.2 to 1.45% in the presence pristine CNTs. The inclusion of pristine CNTs increased the electric conductivity of the aramid films with a minimum threshold value at the loading of 3.5 wt % of CNTs. Such mechanically strong and thermally stable aramid and easily processable composites can be suitable for various applications including high performance films, electromagnetic shielding and radar absorption.